1. Field of the Invention
The present invention is related to an autoconverter which is a clock-control frequency converter that is supplied with an input voltage from a DC voltage source and comprising at least one transistor that can be switched on and off by turn-on pulses of a control circuit in the timing of a switching frequency and comprising a device for current limitation which operates on the basis of current-dependent, chronological shortening of the turn-on pulses, and comprising a further device for further limitation that is effective in an upper range of the current to be limited.
A converter of the type set forth above is already known from the German application B1 28 38 009. In this known converter, a power switching transistor is driven with turn-on pulses of constant working frequency by a clock generator. For controlling the output voltage, the pulse duty factor, i.e. the quotient of the on time of the power switching transistor relative to the cycle duration, is varied. Given an overload at the output of the autoconverter, the current in the power circuit is limited for protecting the components. This is achieved by shortening the on time. The switching transistor is thereby inhibited with the assistance of an aremeter circuit as soon as the measured, momentary value of the currents flowing through the switching transistor exceed a given limit value.
Since the limited switching speed of the switching transistor and the operating times of the measurement and control circuit cause a minimum on time, and this minimum on time would require an economically-unjustifiable over-dimensioning of the components of the power circuit in case of a short circuit, a further aremeter circuit engages in the known converter if the minimum on time of the switching transistor must be downwardly transgressed. In the known converter, this occurs when the voltage at the working impedance of a current transformer has therefore risen by more than the threshold voltage of a diode. This additional action, repeated as necessary, introduces switching pauses having the duration of a plurality of cycles, so that the current in the power circuit does not rise above a prescribed limit. A farther-reaching current limitation can be achieved in this manner. However, the second measuring circuit is only activated after the current has risen to a value that lies clearly above the limit value of the first measurement circuit.
In a current limitation on the basis of omitting individual turn-on pulses, periodic whistling noises and/or operating conditions can occur over and above this wherein the converter continuously oscillates between two operating points. Furthermore, difficulties in a defined function of such a current limitation can result in that a plurality of tolerances are relevant for the function, namely tolerances of the comparator of the second stage, tolerances of the diode threshold voltage, tolerances of the reference, temperature response of the evaluation circuit and a scatter of the plurality of suppressed pulses.
Given high-performance apparatus with a low internal resistance or given apparatus having a comparatively-high output voltage, this minimum on time in the short at the output side can result in an output current that lies clearly above the desired reference value and therefore inadmissibly loads the components in the output circuit.